1. Technical Field
The present invention relates to a power supply unit that supplies power to a measuring instrument for measuring various data of plant equipment or a watching device at a site where various plants are located.
2. Background Art
In a large-scale plant using electrical equipment, such as electric motors or transformers, for example, various plant facilities, such as power plants or water treatment plants, and the like, it is essential to secure reliability of the plant and improve rate of operation. That is, in such a large-scale plant, data of the set-up electrical equipment is collected to serve as information concerning operation, and operating conditions are subject to watch to immediately cope with anything abnormal that has been detected.
For such purposes, it is a recent trend to introduce an equipment condition-watching device for watching the conditions of the equipment so that various data are measured from the electric-power equipment, electrical equipment and the like installed in the plant. The data are then observed, and conditions of the equipment are watched or analyzed on the basis of the contents thereof.
The equipment condition watching device for watching the conditions of the equipment is directed not only to machine/electrical equipment such as electric motors, electric generators, pumps, valves, and piping but also to calculators and printed boards built into the equipment and measuring instrument.
There have been conventionally two types of such equipment condition watching devices for watching the conditions of the equipment. One is a device that is located close to the electrical equipment to be watched. This device measures data of the equipment to be watched regularly or at arbitrary times, thereby watching the conditions of the equipment at the time of measurement in substitution for a patrol carried out by an operator in the plant (the device is hereinafter referred to as a detecting device). The other is a device that measures data of the equipment to be watched on line at all times via a plurality of the detecting devices, thereby watching the conditions at all times (the device is hereinafter referred to as a central control device). The detecting device and the central control device are both located, e.g., on the site of the same plant. However, several detecting devices are widely placed in a scattered manner in the so-called site, while the central control device is located in a centralized manner in the so-called control building or the like. Both of them are located within a relatively short distance from each other. As a result, in order to transmit the collected data, a method of transmission without construction of a power line such as radio system can be easily employed.
Now, in order to operate the detecting device, a power supply is necessary as a matter of course. However, most of the detecting devices usually belong to a category of so-called light electric appliances and are operated at an extremely low voltage and a very small current. On the other hand, there is a lot of electrical equipment to be watched which can be operated at a high voltage and a large current. Therefore, it is often the case that although the device to be watched is an electrical equipment, an appropriate power supply for the detecting device is not available at the site of installation.
As a matter of course, it may be said possible to obtain a low voltage from the electrical equipment to be watched by a well-known method such as using a voltage-reducing resistance or transformer. Actually, however, for the purpose of securing voltage endurance, cost effectiveness is exceedingly bad, and moreover it is difficult to prevent invasion of a surge voltage resulting in deficiency in practical use. Thus, to provide a power supply, it becomes necessary to lay a power supply cable additionally as a power supply for measurement, or to employ a battery; otherwise electric power must be generated at the site utilizing, e.g., a solar battery. However, in order to lay and connect the power supply cable to the detecting devices widely scattered in the plant, a large cost is required. Moreover, the battery may bring about the trouble of maintenance or replacement. As for the solar battery, battery life is restricted, and a place for installation is so restricted that the solar battery cannot be used in this field of art.
As a method to solve these problems, Japanese Patent Publication (unexamined) No. Hei. 6-58960 proposed a power supply unit.
FIG. 9 is a schematic diagram of a known power supply unit which is similar to that disclosed in the above-mentioned gazette and which obtains a power via a current transformer used as a power supply for watching a transmission line. FIG. 10 is specific diagram of such a circuit. In FIG. 9, reference symbol A designates a power supply unit. Reference numeral 81 designates a power line such as a distribution line. Numeral 82 designates a current transformer taking a power supply power from the power line 81 in a non-contact manner. Numeral 83 designates an AC current control circuit. Numeral 84 designates a rectifier circuit. Numeral 85 designates a voltage detection/control circuit.
The power taken from the current transformer 82 needs to be supplied as a stable power supply. This power control means is comprised of the AC current control circuit 83, the rectifier circuit 84 and the voltage detection/control circuit 85 in FIG. 9, and a circuitry thereof is specifically shown in FIG. 11. In the drawing, a resistance 86 in the AC current control circuit 83 and a condenser 87 form a protective circuit for protection when an abnormal voltage such as surge is generated. Numeral 88 designates a triac. Numerals 89a, 89b are phototransistors. The rectifier circuit 84 is a full-wave rectifier circuit, and an output thereof is connected to circuits such as constant voltage diode 90 and light-emitting diodes 91a, 91b for protection against over voltage in the voltage detection/control circuit 85. Thus, when a DC output from the rectifier circuit 84 exceeds an inverse voltage of the constant voltage diode 90, a current flows through the light-emitting diodes 91a, 91b to cause them to emit light. By means of these signals of light, the phototransistors 89a, 89b are operated to feed a current through a gate of the triac 88. By this current, an anode and a cathode are brought into short circuit causing an input of AC to be interrupted. However, the triac 88 will be recovered by means of zero cross point of AC and will supply power up to a next over voltage.
The conventional power supply unit is arranged as mentioned above so as to control Ac. Therefore, even if the primary current varies, any over voltage is not outputted. However, the voltage detection/control circuit 85 operates at the moment when a value of a voltage waveform reaches to a voltage V90 of the constant voltage diode 90, and the AC control circuit 83 operates so as to surpass the output voltage. Therefore, as shown in FIG. 11, as to the output waveform, duration of the waveform becomes smaller as the primary current augments. As a result, as shown in FIG. 12, an average value of the output voltage shows such a characteristic as to be lowered as the primary current augments, above a level of the operation of the triac. In FIG. 11, numeral 93 is a waveform of the output voltage of the transformer 82 when the primary current is small. Numeral 94 is a waveform of the output voltage of the transformer 82 when the primary current is large. Numeral 93a is a waveform of the output voltage after operating the triac 88 on the waveform 93 thereby interrupting the waveform. Numeral 94a is a wave form of the output voltage after operating the triac 88 on the waveform 94 thereby interrupting the waveform.
Therefore, the range of the primary current magnitude sufficient for obtaining a stable voltage output is narrow. Even when employing a circuit for stabilizing voltage as post-treatment, the voltage cannot be perfectly restrained from fluctuating. After all, performance as a power supply of an instrumentation circuit for carrying out a precise measurement is not sufficiently exhibited, and, moreover, such a conventional power supply unit cannot be used for any power line subject to a load, such as an electric motor of which current fluctuates significantly.
In the conventional power supply unit having the above-mentioned arrangement, although cable laying is not required, a problem exists in that a range of the primary current in which a stable output voltage can be obtained is exceedingly narrow, and, moreover, another problem exists in that it is difficult to obtain a stable voltage as a power supply for instrumentation.
The present invention was made to solve the above-discussed problems and has an object of providing a power supply unit that operates stably over wide range of variation in primary current and has a stability of such a level as to be capable of driving a computer at the site, as a power supply of a precise measuring instrument even in the case that the source of the primary current varies over a wide range, for example, in case of an electric motor.
A power supply unit according to the present invention comprises:
a current transformer mounted on a power supply line of an electrical equipment;
a rectifier circuit connected to an output side of the mentioned current transformer;
a condenser connected via a reverse current inhibit element to an output terminal of the mentioned rectifier circuit and charged with an output current from the mentioned current transformer;
a voltage watching circuit for outputting a command signal when a voltage at both terminals of the mentioned condenser is over a predetermined first voltage, and for stopping the mentioned command signal when the mentioned voltage at both terminals is below a predetermined second voltage; and
a switching circuit for connecting a resistor to the output terminal of the mentioned rectifier circuit in response to the mentioned command signal, and reducing a current for charging the mentioned condenser.
As a result, a voltage can be taken from the power supply line in a non-contact manner, and also a stable voltage can be taken with respect to a wide range of variation in the primary current.
The power supply unit according to the invention further comprises a constant voltage regulator for obtaining a predetermined constant voltage output from the voltage at both terminals of the mentioned condenser.
As a result, an extremely stable voltage can be outputted.
In the power supply unit according to the invention, it is preferable that a plurality of mentioned switching circuits and a plurality of mentioned voltage watching circuits are provided respectively, and the mentioned first predetermined voltage set at each of the mentioned plurality of voltage watching circuits is different from each other.
As a result, a stable voltage can be outputted in a wider range of variation in the primary current.
In the power supply unit according to the invention, it is preferable that the mentioned current transformer is a current transformer including a core surrounding the mentioned power supply line and a secondary winding wound around the mentioned core.
As a result, a power supply can be taken in a non-contact manner.
In the power supply unit according to the invention, it is preferable that the mentioned core is composed of a saturable reactor being magnetic-saturated in one cycle of a primary current of the mentioned current transformer.
As a result, a stable output voltage can be obtained in the wider range of variation in the primary current.
The power supply unit according to the invention further comprises a resistor connected in parallel with a secondary winding of the mentioned current transformer; and wherein the mentioned rectifier circuit is a voltage multiplying rectifier or Cockcroft-Walton type step up circuit.
As a result, a stable output voltage can be obtained even in the range of low primary current.
In the power supply unit according to the invention, it is preferable that the mentioned first predetermined voltage is set to be close to a maximum voltage at which the mentioned constant voltage regulator can be operated, and that the mentioned second predetermined voltage is set to be close to a minimum voltage at which the mentioned constant voltage regulator can be operated.
As a result, a stable constant voltage output can be obtained.